Math

Desmos.com's calculator is a great tool for anyone in high school or university who is studying any subjects related to maths. The online graphing webpage will take any function you give it and display a clean graph. You can zoom in and out of the graph as much as you want and there is a helpful mathematical keyboard at the bottom of the screen to input more complex function symbols.

Computational knowledge engine — solve equations, plot functions, convert units, look up chemical compounds, analyze data. Like Google but for computation.